A plasma process system which processes the surface of a substrate, such as a semiconductor wafer, using a plasma is used in a fabrication process for a liquid crystal display or the like. As plasma process systems, there are, for example, a plasma etching system which performs etching on a substrate and a plasma CVD system which performs chemical vapor deposition (Chemical Vapor Deposition: CVD). Of them, a parallel plate plasma process system is widely used because it has an excellent process uniformity and its system structure is relatively simple.
The structure of a parallel plate plasma process system is shown in FIG. 17. As shown in FIG. 17, a plasma process system 101 comprises a chamber 102, a shower electrode 103 which feeds a process gas into the chamber 102 and constitutes an upper electrode, and a susceptor 104 on which a subject W to be processed, such as a semiconductor wafer, is placed and which constitutes a lower electrode.
The shower electrode 103 comprises an electrode plate 106 having multiple gas holes 105, and an electrode support 108 having a hollow portion 107 which leads the process gas to the gas holes 105. The electrode plate 106 is supported on the electrode support 108 at its peripheral portion by screws or the like and the supported portion is covered with a shield ring 109 made of an insulator. The shield ring 109 has an opening having a smaller diameter than the electrode plate 106 and is constructed in such a way that the electrode plate 106 is exposed to the inside of the opening. The shield ring 109 reduces the generation of abnormal discharge at the supported portion.
The plasma process system 101 feeds a process gas (the solid-line arrows in the diagram) to the to-be-processed subject W through the gas holes 105 of the electrode plate 106 and supplies RF power to the electrode plate 106 to form an RF electric field (the broken-line arrows in the diagram) between the exposed surface of the electrode plate 106 and the susceptor 104. This generates the plasma of the process gas on the to-be-processed subject W and performs a predetermined process on the surface of the to-be-processed subject W.
The plasma process system 101 with the above-described structure has the following problems (1) and (2).
(1) To secure insulation, the shield ring 109 which protects the periphery of the electrode plate 106 is made of a plate-like member having a thickness of, for example, about 10 mm. The electrode plate 106 is placed over the shield ring 109 in such a way as to be exposed to the inside of the opening of the shield ring 109. At this time, a step Δ is formed between the peripheral portion of the exposed surface (bottom surface) of the electrode plate 106 and the surface which is near the opening of the shield ring 109.
Such a step Δ varies the process characteristic of the whole surface of the to-be-processed subject W and reduces the process uniformity. That is, the gas in the opening that is supplied through the gas holes 105 stays at the step Δ, disturbing the flow of the gas. This makes the gas supply at the center portion and end portion of the to-be-processed subject W uneven, thus lowering the process uniformity.
The diameter of the exposed surface of the electrode plate 106 (hereinafter referred to as “upper electrode diameter”) that contacts a plasma is so formed as to be nearly equal to the diameter of the surface of the opposing to-be-processed subject W. That is, the upper electrode diameter is not determined to optimize a gas flow and an electric field, which are formed between the electrode plate 106 and the to-be-processed subject W, and to carry out a process with high uniformity. Therefore, a process with a sufficiently high uniformity may not be executed.
Even in case where the gas blowoff diameter and the upper electrode diameter are changed in order to optimize the gas flow and electric field, the gas blowoff diameter and the upper electrode diameter are substantially determined by the diameter of the opening of the shield ring 109. It is therefore difficult to optimize the gas blowoff diameter and the upper electrode diameter by independently varying them to thereby improve the process uniformity.
As apparent from the above, the conventional plasma process system 101 did not have the gas blowoff diameter and the upper electrode diameter optimized to ensure sufficiently high process uniformity.
(2) Dry cleaning using a halogen-based gas, such as fluorine-based gas, is performed in the plasma process system 101. Specifically, a halogen-based gas is generated inside or outside the chamber 102 and a film adhered or deposited to the inside of the chamber 102 is removed by a halogen active seed (e.g., fluorine radicals) in the gas plasma Particularly, fluorine has a high reactivity to silicon and is suitable to clean a process system which processes silicon-based films.
Here, to avoid metal contamination, the electrode plate 106 is formed of silicon. Such a silicon-based electrode plate 106 is likely to be etched by the cleaning. Particularly, in remote plasma cleaning which generates the plasma of the cleaning gas outside the chamber 102 and selectively supply a radical seed in the chamber 102, the radical seed is highly active so that the degradation (etching) of the electrode plate 106 becomes noticeable.
The degradation of the electrode plate 106 means a change in the shape of the electrode plate 106 and changes the RF electric field. A change in electric field varies the process characteristics at, for example, the center portion and end portion of the to-be-processed subject W, thus lowering the process uniformity.
In case where the electrode plate 106 formed of silicon is used, as mentioned above, cleaning is likely to etch the electrode plate 106 so that a process with sufficiently high uniformity may not be carried out.